Threaded pipe joints are widely used in connecting steel pipes used in oil-producing industrial installations such as oil well pipes. For the connection of pipes used in oil or gas prospecting and production, standard threaded pipe joints based on the API (American Petroleum Institute) standard are typically used.
Since crude oil wells and natural gas wells have recently increased in depth, and horizontal wells and directional wells are now more common than vertical wells, excavation and production environments are increasingly under harsh condition. Furthermore, an increase in well development under hostile environments, such as in ocean and polar regions, has led to diversified performance requirements for threaded pipe joints, such as compression resistance, bending resistance, and sealability against external pressure.
On the other hand, in order to reduce the amount of excavation during the development of a well, the well needs to be made slimmer. Among high-performance special threaded pipe joints called premium joint, requirements for an integral-type threaded pipe joint that directly connects pipes with no coupling member interposed therebetween are increasing.
The premium joint normally has a tapered thread, a metal-to-metal seal portion, and a torque shoulder portion at the end of each pipe. These are components forming each of a pin that is a male-shaped portion provided at one end of a pipe and a box that is a female-shaped portion provided at the other end of the pipe and is screwed or fitted on the male-shaped portion. These components are designed such that when a joint (which means a threaded pipe joint, the same applies hereinafter) is tightened, female and male components having the same name face each other.
The tapered thread is important for firmly securing the joint. The metal-to-metal seal portion is important for ensuring sealability by bringing the box and the pin into metal-to-metal contact with each other in the region of the metal-to-metal seal portion. The torque shoulder portion serves as a shoulder face that acts as an abutment during the make-up of the joint.
In the integral-type threaded pipe joint (hereinafter also referred to as integral joint), one or two or more metal-to-metal seal portions are provided in the axial direction (which means the axial direction of the pipe, the same applies hereinafter). At least one of the metal-to-metal seal portions is provided on the outer peripheral surface of an unthreaded portion (hereinafter referred to as nose) continuous with the pin-front-end-side thread end of the tapered thread of the pin, and on the inner peripheral surface of an unthreaded portion (hereinafter referred to as nose hole) continuous with the box-back-end-side thread end of the tapered thread of the box. When make-up the joint, the metal-to-metal seal portion of the nose and the metal-to-metal seal portion of the nose hole come into contact with each other in the radial direction, and this metal-to-metal seal portion forms a seal surface (referred to as inner radial seal surface for convenience sake) that prevents fluid inside the pipe from entering the region of the tapered thread.
In some integral joints, in each of the pin and the box, the region of the tapered thread is divided into two parts in the axial direction. Of the two parts, the thread row on the pin-front-end side and the thread row on the box-back-end side engaged with this are referred to as first thread row. On the other hand, the thread row on the pin-back-end side and the thread row on the box-front-end side engaged with this are referred to as second thread row. In the radial direction (which means the radial direction of the pipe, the same applies hereinafter), the first thread row is on the inner side, and the second thread row is on the outer side. The torque shoulder portion is provided at the boundary of the first thread row and the second thread row, and this is referred to as intermediate shoulder. If this intermediate shoulder is provided, since the shoulder surfaces of the pin and the box come into contact with each other at the time of tightening, a tightening torque is increased. Therefore it is possible to watch the tightening torque for checking whether the female and male threads have been appropriately fitted with seal surfaces.
In an integral joint having the intermediate shoulder, when two metal-to-metal seal portions are provided in the axial direction, one of the two metal-to-metal seal portions forms the inner radial seal surface. The other metal-to-metal seal portion is provided on the outer peripheral surface of the unthreaded portion continuous with the back end of the second thread row of the pin (referred to as pin-back-end-side unthreaded surface for convenience sake), and on the inner peripheral surface of the unthreaded portion continuous with the front end of the second thread row of the box (referred to as box-front-end-side unthreaded surface for convenience sake). When make-up the joint, the metal-to-metal seal portion of the pin-back-end-side unthreaded surface and the metal-to-metal seal portion of the box-front-end-side unthreaded surface come into contact with each other in the radial direction, and this metal-to-metal seal portion forms a seal surface (referred to as outer radial seal surface for convenience sake) that prevents fluid outside the pipe from entering the region of the tapered thread.
A threaded joint for pipes (threaded pipe joint) described in Patent Literature 1 is shown in FIG. 5 as an example of conventional art of an integral joint having the intermediate shoulder. The aim (object) of the invention described in Patent Literature 1 is to produce a threaded joint for pipes that maintains appropriate rigidity and that is provided with an improved seal, to improve the structural resistance (characteristic) of the joint to a high load, specifically to a compressive load, and to prevent the characteristic from affecting the seal function. In the invention described in Patent Literature 1, a reinforcing section protruding from the metal-to-metal seal portion of the box-front-end-side unthreaded surface to the frontmost end of the box is provided, the length, or the length and the wall thickness of this reinforcing section are regulated, and the total length of the reinforcing section of the box is prevented from coming into contact with the opposing pin-back-end side pipe (pipe body portion).